Conventional method for the recovery of the eyesight (refraction) of a patient of aphakia caused by the extraction of crystalline lens for treating, for example, cataract include correction with glasses, the use of contact lenses and the transplantation of intra-ocular lenses.
Although the correction with glasses enables the recovery of the eyesight after the operation, this method is accompanied by some troubles such as the constriction of visual field (enlargement of retinal image) and a so-called Jack-in-the-box phenomenon. In order to use glasses in practice, therefore, a certain period of time for training is required. In the case of hemiaphakia, in particular, aniseikonia makes it impossible to achieve binocular functions.
Contact lenses are effective on such aniseikonia. Recently soft contact lenses of a high moisture content, which can be continuously worn, have been developed and thus the aforesaid problem is now being settled. For many aged patients, however, it is difficult to handle contact lenses. Thus there are few patients who wear contact lenses provided after the operation in practice.
Accordingly, it seems that correction of eyesight with glasses or contact lenses is not preferable.
On the other hand, the transplantation of artificial crystalline lens (i.e., an intra-ocular lens), which has been performed for these 30 years, is superior in various points, for example, showing less enlargement of retinal image, never accompanied by constriction of visual field or annular scotoma, achieving binocular functions, which is advantageous compared with glasses, in particular, in the case of hemiaphakia, requiring no training time, requiring no handling, once transplanted. Owing to recent progress in microscopes and ultrasonic knives, transplantation techniques have been improved. Further, the material and shape of intra-ocular lenses have been improved. Thus it is expected that the aforesaid intra-ocular lenses will become the most important method for the correction of eyesight of patients of aphakia.
Thus, an intra-ocular lens shows an excellent effect of correcting eyesight. However, it is a foreign matter in an eye and thus suffers from the problem of ophthalmicus complicating diseases. It is sometimes observed that the transplantation of an intra-ocular lens causes corneal endothelium disorders followed by decompensation, which finally results in blindness. Therefore, an intra-ocular lens should be made of a material which shows no biotoxicity and a high biocompatibility and is never modified or deteriorated in a living organism.
Natural light involves rays falling within the ultraviolet, visible and infrared wavelength regions. There is a fear that the penetration of a large amount of ultraviolet rays through an eye might induce retinal disorders. The crystalline lens of an eye further plays a role of predominantly absorbing the ultraviolet rays so as to protect the retina. Thus, the transmission of ultraviolet rays is a serious problem in the aforesaid aphakial eyes. Therefore, it is required that the material of the aforesaid intra-ocular lens would absorb ultraviolet rays falling within a wavelength range of from 200 to 380 nm and remain transparent for visible rays falling within a wavelength range of from 380 to 780 nm. It is furthermore desired that the material essentially has such a small specific gravity as to lighten a burden of the eye as well as such a high refraction index as to minimize the thickness of the lens.
Polymethyl methacrylate (PMMA), which is now the most frequently employed for intra-ocular lenses, is excellent in optical properties, highly resistant against acids, alkalis and organic solvents and scarcely shows any change with the lapse of time.
However, PMMA shows a low heat stability (i.e., a glass transition temperature (Tg) of 105.degree. C. or lower), which makes it impossible to sterilize PMMA with a steam autoclave. That is, autoclave sterilization is usually performed at 121.degree. C. under 1.2 atmosphere for 1 hour. Under such conditions, PMMA would be softened and deformed, which makes it unavailable. Thus, an intra-ocular lens made of PMMA is sterilized with the use of, for example, ethylene oxide gas. In this case, however, it is liable that the gas remaining in the lens would induce inflammation on a mucosa, when the intra-ocular lens is put into an eye. Thus, the aforesaid gas sterilization method necessarily involves a degassing step. This step requires approximately 2 weeks, which results in an increase in the cost. As a result, intraocular lenses sterilized by this method are generally more expensive than those subjected to steam sterilization. In addition, PMMA transmits a substantial amount of ultraviolet rays, which causes another problem of the damage of the retina with ultraviolet rays, as described above. In order to solve this problem, an UV absorber to PMMA is proposed, as described in JP-A-60-232149. (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However this method is not a preferable one, since the addition of an UV absorber would deteriorate the transmittance of visible rays. Further, there is a fear that the UV absorber thus added might slowly ooze out from the lens and thus exert undesirable effects to an organism. In addition, the aforesaid PMMA has a lower refraction index (approximately 1.49) than that of glass. Thus there is a possibility that the lens would adhere to the iris and thus cause complicating diseases.
As described above, PMMA has a number of disadvantages, though it is advantageous in many points. Thus, attempts have been made to develop a material which can be sterilized in an autoclave, absorb ultraviolet rays and has a high refraction index. For example, glass, which has a high refraction index and can absorb ultraviolet rays, is difficult to process. Further, it has a large specific gravity (2.5) and thus a lens made of glass is heavy per se, which causes a serious burden on an eye. Thus glass is not suitable as a material for an intra-ocular lens. Natural and synthetic crystalline materials, for example, sapphire, ruby, corundum, silicon, diamond can also absorb ultraviolet rays. However, these materials are unsuitable as a material for intra-ocular lenses, since it is difficult to process them and each of them has a large specific gravity, similar to glass. Recently, therefore, synthetic resins capable of substituting for PMMA have attracted public attention and attempts have been made to utilize, for example, polysulfone, polyarylate, polyether imide therefor. The above-mentioned polysulfone has a high refraction index, can absorb ultraviolet rays and shows a softening point of 175.degree. C. which enables autoclave sterilization. However the poor processability of polysulfone makes the practical use thereof impossible. Polyarylate has a high refraction index, can absorb ultraviolet rays and can be sterilized in an autoclave. However, the poor processability of polyarylate prevents the practical application thereof, similar to the case of the above-mentioned polysulfone. On the other hand, polyether imide is excellent not only in refraction index, ultraviolet ray absorptivity and autoclave sterilization properties but also in processability. However, this resin has a yellow or yellowish brown color, which restricts the transmission of visible rays. Thus, it is impossible to use polyether imide as an intra-ocular lens.
Namely, although PMMA has such disadvantages as described above, no material substituting therefor has been found out so far. Thus, PMMA, which has been sterilized by the aforesaid expensive gas sterilization method and contains an UV absorber which might exert some undesirable effects on living organisms, is employed as a material for intra-ocular lenses at present.
Therefore, it has been urgently required to develop an intra-ocular lens material which can be easily processed into a thin lens by, for example, mechanical processing or molding, has a specific gravity of 1.7 or less, preferably 1.5 or less, and a refraction index of 1.5 or more, preferably 1.6 or more, is chemically stable and biologically compatible, can absorb ultraviolet rays which is dangerous to the retina and has such a high heat resistance as to withstand autoclave sterilization.